The invention relates to the field of video systems, and in particular to a circuit for controlling luminance signal amplitudes for application in a video system.
Video and television systems generally have an automatic luminance control (brightness control), which controls the average overall brightness in dependence on the brightest positions of a picture. The primary purpose of automatic luminance control is to protect the picture tube by limiting the beam current.
In picture-in-picture (PIP) systems, two pictures are displayed simultaneously on one television screen by overlaying a smaller, secondary picture on a main picture. A problem with PIP systems is that a bright secondary picture often causes the automatic luminance control system to reduce the brightness of the main picture. As a result, the picture is properly adjusted for the brightness of the secondary picture, but this adjustment is often too dark for the main picture.
Therefore, there is a need for a circuit for controlling luminance signal amplitudes to control the brightness of the image in the main picture effectively independently of the brightness of the image in the secondary picture.
It is an object of the present invention to create a circuit to control the luminance signal amplitude in video systems, and thus picture brightness. Especially in the case of picture-in-picture systems, the inventive circuit controls the brightness of the main picture, independent of and uninfluenced by the brightness of a secondary or mini-picture which is cut into the main picture.
The first device may include a first counter to count the pixels in a line which exceed a prescribed luminance threshold, and a second counter to count the lines in a picture, in which the status of the first counter exceeds a certain pixel-number threshold. The first device may also include a third counter to count the pictures in which the status of the second counter exceeds a line-number threshold.
The invention is especially based on the insight that the picture shown on a screen is shown too bright (i.e., a viewer perceives it as too bright), if more than a certain number of pixels have a luminance which is greater than a specific peak value, and if this condition prevails in more than a certain number of lines in a picture and in more than a certain number of successive pictures.
The circuit is preferably implemented as a digital circuit, and as part of the (digital) picture-in-picture subassembly.
The first device may include a compare the luminance of a pixel with a luminance threshold, the first comparator then actuating the first counter. The first device may also include a second comparator to compare the counter status of the first counter with the pixel number threshold, the second comparator then actuating the second counter. In addition, the first device may include a third comparator to compare the counter status of the second counter with the line number threshold, the third comparator then actuating the third counter, in such a fashion that its status is incremented by one if the status of the second counter exceeds the line number threshold, and is decremented by one if the status of the second counter falls below this threshold.
The second device preferably includes a fourth comparator and a unit to create incrementally increased or reduced luminances, if the counter status of the third counter exceeds a first limit value FLDO or falls below a second limit value FLDU. In particular, the first limit value FLDO can have a value +2, and the second limit value FLDU can be negative.
The luminance threshold and the first two limit values FLDO and FLDU preferably are set through a bus, so as to afford optimal adaptation to the TV system.